ABSTRACT Ataxin-2 (ATXN2) is a ubiquitously expressed RNA-binding protein (RBP) conserved across eukaryotic species from yeast to human. Trinucleotide (CAG) repeat expansion mutations of the poly-glutamine (polyQ) tract of the ATXN2 gene are associated with several neurodegenerative diseases including spinocerebellar ataxia 2 (SCA2) and amyotrophic lateral sclerosis (ALS), a fatal adult-onset disorder characterized by the progressive death of motor neurons of the brain and spinal cord. In vivo models recapitulate this link to motor neuron degeneration and ALS, as ataxin-2 is reported to be a toxic modifier of TDP-43, a protein most commonly associated with both the familial and sporadic forms of ALS. Studies have supported this toxic gain-of-function model, as reduction of ataxin-2 levels seems to serve a protective role against neuron degeneration while simultaneously prolonging survival in Atxn2-null mice in the presence of TDP-43 overexpression. At the same time, endogenous neural- specific functions of ataxin-2 are still poorly understood, as are its direct role in mediating ALS pathogenesis. This project aims to uncover the transcriptome-wide regulatory role of ataxin-2 in the central nervous system in order to mechanistically characterize the interaction between ATXN2 mutations and motor neuron death. This proposal seeks to use molecular and genomic techniques, including eCLIP-seq, RNA-seq and ribosome profiling, to map the physical and functional ?interactome? of ataxin-2 in mouse brain and spinal cord, as well as in human iPSC-differentiated motor neurons. To address the connection to motor neuron disease, this proposal also aims to generate an isogenic iPSC model using CRISPR/Cas9 technologies by knocking-in ALS-associated polyQ expansion sequences into the ATXN2 genomic locus. These cell lines will then be differentiated into mature motor neurons in order to study neural-specific transcriptional and translational perturbations introduced through repeat expansions. This study will directly characterize the role of ataxin-2 in neuron-specific RNA metabolism, as well as identify specific target genes disrupted with polyQ mutations. If successful this research will define the underlying molecular mechanisms linking ataxin-2 to ALS risk, identify novel disease-relevant pathways that potentially serve as targetable avenues for therapy, and provide a broadly applicable disease modeling strategy to investigate the direct genetic influence of other repeat expansion mutations in future work.